During this test, the engine must be warmed up to normal operating temperature, and the battery must be completely
1. Before removing the spark plugs, clean the area around them (use compressed air if available, otherwise a bicycle pump or a small brush can be used). The goal is to prevent dirt from getting inside the cylinders when checking.
2. Remove all spark plugs from the engine (see chapter Car maintenance).
3. Lock the throttle fully open or have an assistant hold the pedal down.
4. On carburetor models, disconnect the low voltage wire from the ignition coil. On fuel injected models, disable the fuel pump and ignition circuit by removing the main relay (indicated by an arrow).
5. This is to prevent ignition of fuel that may have been injected into the engine compartment. Usually the main relay is located under the hood near the fuse panel, however, to determine the exact location on your model, refer to the Head Onboard electrical system.
6. Install a compression gauge in the spark plug hole No. 1 (cylinder number 1 is located on the side of the radiator).
7. Use the starter to crank the engine through at least seven compression strokes and observe the compressor readings. On a healthy engine, the compression ratio will increase rapidly. A low compression ratio in the first stroke with a gradual increase in subsequent strokes indicates wear on the piston rings. A low compression ratio in the first stroke that does not increase in subsequent strokes indicates valve leaks or burnt cylinder head gaskets (a crack in the cylinder head can also be the cause). Carbon deposits on the inside surfaces of the valve heads can also cause low compression. Record the highest measured value.
8. Repeat the operation for the remaining cylinders and compare the results with the compression ratios in Table of sizes and adjustment data at the end of the Guide.
9. If the compression ratio is low, pour a small amount of engine oil through the spark plug holes into each cylinder (by pressing approximately three times on the plunger-type oiler) and recheck.
10. If after adding oil the compression ratio has increased, then the piston rings are definitely worn out. If the compression ratio has not risen noticeably, then there is a leak in the valves or through the cylinder head gasket. Leaking valves can be caused by burnt valve seats and/or bevels, bent valves, cracks and dents.
11. If two adjacent cylinders have the same low compression ratio, then it is likely that the cylinder head gasket between them has burned out. The presence of a coolant in the combustion chambers or crankcase will confirm this suggestion.
12. If the compression ratio in one of the cylinders is 20% lower than in the others, and the engine idling is slightly uneven; the camshaft camshaft may be worn out.
13. If the compression ratio is unusually high, then the combustion chambers are probably covered with soot.
14. In this case, remove the cylinder head and remove carbon deposits.
15. If the compression ratio is below normal or is very different between the cylinders, then it is useful to diagnose leaks in an auto repair shop. This check will accurately indicate the cause of the leaks and its extent.